Modern cable television systems have evolved over many years to provide enhanced services as new technologies have emerged. Whilst the same network infrastructures and topologies are used additions such as optical fibre have been deployed to improve performance. Digital technologies have also made a significant impact in recent years and have been added compatibly to existing analogue systems offering better exploitation of the network resources and the introduction of new services. One such new service type is Video on Demand [VOD] which allows viewers to select specific content from a list and have it streamed to them for immediate viewing.
However the delivery of such a new service places new demands on the management of the network and its resources whose basic traditional structure is defined by fixed assets that convey a set of pre-determined content packages to viewers. This is achieved by means of a delivery network having several components:                Content storage resources (video servers) at one or more network nodes,        a core network to connect such video servers to access hubs (such as local cable head ends),        an access network that connects consumers with the access hubs, and        network management resources to control the delivery of services.        
The access network can be for example one or more cable networks, which typically use a wide frequency band divided into RF channels each of which carries an analogue or digital signal. The analogue channels carry one service per channel but the digital ones carry a multiplex using Quadrature Amplitude Modulation [QAM] comprised of several services carried by separate digital streams. Normally all viewers' receivers are able to receive all the RF channels [subject to any controlled access service constraints] and hence all services regardless of where they are located geographically in the local area. However not all viewers gain their service from the same head end hardware resource. In small systems this may be possible but in more typical systems, especially those serving large communities, the viewers are grouped, e.g. geographically, so that they share hardware resources in small numbers appropriate to service management criteria. The viewer uses an on-screen schedule list, the electronic programme guide, to see the available services and makes a selection from it. The receivers locate each requested service by means of a combination of RF channel selection and digital content stream identity. Typically the normal analogue and digital TV services and the new VOD services are carried in separate identifiable RF channels some of whose QAM modulation equipment at the head end is dedicated to VOD.
A traditional large cable TV network typically offers a fixed set of broadcast services thus requiring known and substantially fixed network resources. The introduction of VOD services into such a traditional network raises two requirements: the provision of additional variable capacity in the core network to deliver the requested on-demand services and resources in the local network to convey the requested content to the viewers. Despite the use of IP based technology in the core networks, the capacity in these core networks to be allocated to the cable TV service is typically limited and may not necessarily allow for dynamically variable capacity. It may be limited by physical capacity limitations of the core network, or by a need for some of the capacity to be allocated by the cable service provider to other non VOD services. In cases where the core network is provided by a third party, then there may be an agreement which limits how much capacity is allocated to the cable service provider. Such agreements are often based on a fixed capacity within which the cable TV service providers must manage their services.
Thus, it is possible that there are times when the access network carries no VOD services at all [when no viewer attached to it has requested VOD] and other times when demand from viewers in the access networks reaches a peak. The allocation of bandwidth for VOD in the core and access networks must be large enough to accommodate the expected peak demand.
In modern digital cable systems offering VOD services, there is a resource manager often called a Session Resource Manager [SRM] which maintains an activity map of the system and is responsible for allocating resources, including QAM bandwidth, for the delivery of an on-demand request originated by a particular viewer. For each new VOD request the SRM identifies an available Video Server that contains the requested content and a free VOD QAM channel that is available to the viewer such that all of the following constraints can be met:                The Video Server has the chosen content.        The Video Server has sufficient spare bandwidth to deliver the chosen content.        The QAM is reachable via the core network from the Video Server.        The QAM can reach the user's service group in the user's access network.        The QAM has sufficient spare capacity for the bit rate of the chosen content.        